Electrostatic chuck, and method of and apparatus for processing sample

ABSTRACT

An electrostatic chuck includes a pair of electrodes having different polarities; and a dielectric film, formed on top surfaces of the pair of electrodes, on which a sample is to be electrostatically attracted and held when a DC voltage is applied between the pair of electrodes; wherein the respective amounts of electric charges stored on attracting portions of the dielectric film corresponding to the pair of electrodes, directly before stopping supply of the DC voltage applied between the pair of electrodes, are substantially equal to each other. With this chuck, the electric charges stored on the attracting portions of the dielectric film after stopping supply of the DC voltage can be eliminated due to the balance between the electric charges having different polarities. The electrostatic chuck is subjected to a significantly reduced residual attracting force.

BACKGROUND OF THE INVENTION

The present invention relates to an electrostatic chuck, and a method ofand an apparatus for processing a sample using the chuck. The inventionrelates particularly to an electrostatic chuck suitable forelectrostatically holding a sheet-like sample, such as a semiconductorsubstrate or a liquid crystal substrate, when the sample is processed orcarried, and a method of and an apparatus for processing a sample usingthe chuck.

An example of a bipolar type electrostatic chuck using a pair ofelectrodes having different polarities, in the form of anelectrostatically attracting apparatus having a pair of semi-circular orconcentrically-circular shaped flat electrodes, is disclosed in JapanesePatent Laid-open No. 64950/1982. According to this document, byincreasing the ratio between the area of the electrostaticallyattracting apparatus and the electrode area of the pair of flatelectrodes, mounting a substance on the pair of flat electrodes throughan insulator having a thickness of 50 to 200 μm, and electrostaticallyattracting the substance by applying a voltage between the flatelectrodes, it is possible to employ the electrostatically attractingapparatus to hold both a conductive substance and a conductive substancewhose surface is covered with a thin insulating film to provide astronger attracting force and to simplify the structure of theapparatus. The document also describes that the attracting force ismaximized when the areas of the pair of positive and negative electrodesare equal to each to other. Also, another bipolar type electrostaticchuck is disclosed in Japanese Patent Laid-open No. 120329/1994.

The method of holding a sample, for example, a wafer using such anelectrostatic chuck has advantages in that: (1) since the surface of awafer to be processed is not mechanically contacted by the chuck, thewafer can be prevented from being contaminated by abrasive particles andthe like; and (2) since the entire back surface of a wafer is fixedlyattracted on the chuck, the camber of the wafer can be corrected, sothat the contact of the wafer with an attracting surface of the chuckbecomes more reliable when the wafer is finely processed by etching orthe like, to improve the thermal conductivity of the wafer, therebyfacilitating the temperature control of the wafer. For these reasons,the electrostatic chuck is being extensively used as a sample stage(referred to simply as an "electrode") of a dry etching apparatus or aplasma processing apparatus, such as a CVD apparatus.

A bipolar type electrostatic chuck used for a plasma processingapparatus, in the form of an electrostatically attracting apparatus, forexample, in Japanese Patent Publication No. 44747/1982. The documentindicates that a larger attracting force can be obtained during plasmadischarge by making the area of the positive electrode larger than thatof the negative electrode, and that the attracting force in the case ofgeneration of no plasma is maximized when the ratio between the areas ofthe positive and negative electrodes is set at 1.

Another disadvantage of the electrostatic chuck will be described below.In general, to remove a wafer from the electrostatic chuck aftertermination of the processing of the wafer, bar-like supports(generally, called "pushers" or "lift pins") are lifted or pushed upfrom the interior of the electrostatic chuck for pushing up the wafertherefrom. The mechanism involving such bar-like supports is known, forexample, from U.S. Pat. No. 4,565,601 or Japanese Patent Laid-open No.252253/1994. However, in the case where there exists a residualattracting force between an electrostatic chuck and a wafer, if a waferis forcibly peeled from the electrostatic chuck by applying a strongforce against the residual attracting force, there arises a problem thatthe wafer may be cracked or undergo an abnormal discharge sufficientlylarge to destroy devices of the wafer.

To cope with the disadvantages due to a residual attracting force,various methods for eliminating an electric charge stored on anelectrostatic chuck have been proposed. For example, a method ofeliminating an electric charge stored on an electrostatic chuck uponremoval of a sample from the chuck is described in U.S. Pat. No.5,117,121, wherein a residual attracting force eliminating voltage,having a polarity reversed with respect to that of an attracting voltageand which is higher than the attracting voltage, is applied between theelectrodes of the chuck. Another method of eliminating an electriccharge stored on an electrostatic chuck is described in Japanese PatentLaid-open No. 185773/1983, wherein a DC voltage for generating anelectrostatic attracting force is turned off, and thereafter the radiofrequency power for generating a plasma is turned off. Besides these,various methods of removing a sample from an electrostatic chuck aredescribed, for example, in Japanese Patent Laid-open Nos. 112745/1989and 247639/1992.

SUMMARY OF THE INVENTION

The electrostatic chucks described in Japanese Patent Laid-open No.64950/1982 and Japanese Patent Publication No. 4474/1982 have failed toexamine the residual attracting force.

Namely, in the case where the temperature of a wafer as a sample isrequired to be controlled at a specific value during processing of thewafer, for example, at a plasma processing step, a heat transfer gas issupplied between the back surface of the wafer and the electrostaticchuck. For this purpose, the electrostatic chuck has a structure inwhich the wafer mounting surface of the chuck is provided with adispersion groove (called "a gas groove") for uniformly supplying a heattransfer gas. An electrostatic chuck used for holding a wafer subjectedto plasma processing is described, for example, in Japanese PatentLaid-open No. 86382/1995, wherein a wafer mounting surface of the chuckhas a recess for reducing the contact area between the wafer mountingsurface and the wafer, thereby suppressing adhesion of contaminants onthe wafer. With respect to the dispersion groove or recess, variouspatterns have been developed. In the case where a groove or recess isprovided in a wafer mounting surface of an electrostatic chuck asdescribed above, the attracting areas on the positive and negativeelectrode sides change depending on the size and shape of the dispersiongroove or recess, so that a residual attracting force is generated.

Further, even in the case where an electrostatic chuck is used duringplasma processing the, the amounts of the electric charges stored onattracting surfaces on the positive electrode side and the negativeelectrode side are different from each other as a result of generationof a self-bias voltage due to the plasma and the application of a radiofrequency bias, so that a residual attracting force is generated.

As a result, even the bipolar type electrostatic chuck requires a stepof eliminating the electric charge stored on the chuck for removing aresidual attracting force, so that there arises a problem in terms oflowering the throughput in carrying wafers. Another problem is that,since an electric charge remains in a dielectric film constituting anattracting surface of the electrostatic chuck, the attracting surface isliable to attract contaminants which in turn adhere on the back surfaceof a new sample attracted on the attracting surface. In particular, whena wafer is processed using a CVD apparatus which generates depositshaving electric charges, such a problem becomes significant.

Further, the method of eliminating a residual attracting force, asdescribed in U.S. Pat. No. 5,117,121, requires the step of eliminatingan electric charge stored on the chuck by newly applying a reversevoltage. This causes a problem in terms of lowering the throughput incarrying a sample. Also, if the reverse voltage becomes excessivelylarge, there arises another problem in that an electrostatic attractingforce is produced again, to thereby generate a residual attractingforce. Besides, in the method of eliminating a residual attractingforce, as described in Japanese Patent Laid-open No. 185773/1983, since,after stopping of the supply of the DC voltage for electrostaticattraction, supply of the radio frequency power for generating a plasmais stopped, the time required for eliminating the electric charge mustbe made longer. This causes a problem in terms of lowering thethroughput in carrying a wafer. In the case of supplying a heat transfergas to the back surface of a sample simultaneously with electrostaticattraction, supply of the transfer gas is usually stopped upon stoppingthe supply of the DC voltage for electrostatic attraction. As a result,the temperature of the sample is increased and thereby plasma continuousto be produced, so that processing of the sample continues, therebyexerting an adverse effect on the sample, the processing of which shouldhave been terminated.

Additionally, in a plasma processing apparatus, generally, a radiofrequency voltage is applied to a sample stage for controlling anincident energy of ions in a plasma against the sample by means of abias voltage generated at the sample stage. When such a plasmaprocessing apparatus uses a bipolar type electrostatic chuck, it isdifficult to equally apply the bias voltage to the sample resulting fromthe electrode structure of the chuck, as compared with the case of usinga monopole type electrostatic chuck. This possibly exerts an adverseeffect on uniform processing of the sample.

In view of the foregoing, the present invention has been made, and afirst object of the present invention is to provide an electrostaticchuck which is capable of reducing a residual attracting force to avalue in a substantially practically usable range.

To achieve the first object, according to the present invention, thereis provided an electrostatic chuck including: a pair of electrodeshaving different polarities; and, a dielectric film formed on topsurfaces of the pair of electrodes on which a sample iselectrostatically attracted and held when a DC voltage is appliedbetween the pair of electrodes; wherein the amounts of electric chargesstored on attracting portions of the dielectric film corresponding tothe pair of electrodes, directly before stopping the supply of the DCvoltage applied between the pair of electrodes, are substantially equalto each other.

Another object of the present invention is to provide a sampleprocessing method which is capable of reducing the standby time uponremoval of a sample from an electrostatic chuck for improving thethroughput.

To achieve the second object, according to the present invention, thereis provided a sample processing method including the steps of:electrostatically attracting and holding a sample on an electrostaticchuck including a pair of electrodes having different polarities and adielectric film formed on top surfaces of the pair of electrodes, byapplying a DC voltage between the pair of electrodes; and processing thesample electrostatically attracted and held on the chuck through thedielectric film; wherein the amounts of electric charges stored onattracting portions of the dielectric film corresponding to the pair ofelectrodes, directly before stopping the supply of the DC voltageapplied between the pair of electrodes after termination of processingthe sample, are substantially equal to each other, so that the electriccharges stored on the attracting portions of the dielectric film afterstopping the supply of the DC voltage are eliminated due to the balancetherebetween, whereby the sample can be removed from the sample mountingsurface without addition of any special step.

A further object of the present invention is to provide a sampleprocessing apparatus which is capable of reducing the stand-by time uponremoval of a sample from an electrostatic chuck for improving thethroughput.

To achieve the third object of the present invention, there is provideda sample processing apparatus for processing a sample electrostaticallyattracted and held on an electrostatic chuck, the electrostatic chuckincluding: a pair of electrodes having different polarities; and adielectric film formed on top surfaces of the pair of electrodes onwhich a sample is electrostatically attracted and held when a DC voltageis applied between the pair of electrodes; wherein a recess which is notin contact with the back surface of the sample is formed in a surface ofthe dielectric film on which the sample is disposed; and the amounts ofelectric charges of different polarities stored on the attractingportions of the surface of the dielectric film, excluding the recess,are equal to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of one example of a sampleprocessing apparatus using an electrostatic chuck, representing a firstembodiment of the present invention;

FIG. 2 is a sectional view taken on line II--II of FIG. 1, showing theelectrostatic chuck;

FIG. 3 is a sectional view showing the details of a portion III of FIG.1;

FIG. 4 is a timing diagram showing potentials of a wafer and electrodesof the electrostatic chuck shown in FIG. 1;

FIG. 5 is a time chart showing steps of attracting a wafer, processingthe wafer, and eliminating electric charges in the case of processingusing the apparatus shown in FIG. 1;

FIG. 6 is a graph showing a relationship between a residual attractingforce and a time elapsing from the stopping of supply of a radiofrequency voltage to plasma extinction in the case of processing usingthe apparatus shown in FIG.

FIG. 7 is a vertical sectional view of the electrostatic chuck portionshown in FIG. 1;

FIG. 8 is a graph showing a load applied to a wafer upon removal of thewafer from the electrostatic chuck shown in FIG. 7;

FIGS. 9(a) to 9(c) are vertical sectional views showing other electrodearrangements of the electrostatic chuck shown in FIG. 1;

FIG. 10 is a vertical sectional view showing another example ofconnection of a DC power supply for the electrostatic chuck shown inFIG. 1;

FIG. 11 is a timing diagram showing potentials of a wafer and electrodesof the electrostatic chuck shown in FIG. 10;

FIG. 12 is a vertical sectional view showing a further example ofconnection of a DC power supply for the electrostatic chuck shown inFIG. 1;

FIG. 13 is a timing diagram showing potentials of a wafer and electrodesof the electrostatic chuck shown in FIG. 12;

FIG. 14 is a perspective view showing an electrostatic chuckrepresenting a second embodiment of the present invention;

FIG. 15 is a plan view of the electrostatic chuck shown in FIG. 14;

FIGS. 16(a) to 16(c) are diagrams each showing a relationship between agap and an attracting force upon electrostatic attraction;

FIG. 17 is a graph showing a change in resistivity of a dielectric filmof the electrostatic chuck shown in FIG. 14 depending on thetemperature;

FIGS. 18(a) and 18(b) show a sectional view and an enlarged view showinga third embodiment using the electrostatic chuck of the presentinvention, showing a state in which contaminants adhering on adielectric film are transferred on a dummy wafer;

FIG. 19 is a waveform diagram showing another example of removal ofcontaminants shown in FIG. 18, showing a DC voltage applied to theelectrostatic chuck in such a manner as to be alternately changed inpolarity;

FIG. 20 is an elevational view showing a sample processing apparatus asa fourth embodiment using the electrostatic chuck of the presentinvention, in which the electrostatic chucks are used for all waferholding portions of the apparatus;

FIG. 21 is a sectional view showing the details of a wafer holdingportion of a carrying robot in the apparatus shown in FIG. 20;

FIG. 22 is a circuit diagram showing an equivalent circuit of theelectrostatic chuck;

FIG. 23 is a graph showing a relationship between a volume resistivityof a ceramic material and an applied voltage;

FIGS. 24(a) to 24(c) are circuit diagrams each showing an attractingaction and an electric charge eliminating action in the equivalentcircuit shown in FIG. 22; and

FIG. 25 is a graph showing a relationship between a residual attractingforce and a leaving time using an attracting area ratio as a parameter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior to description of the preferred embodiments, the cause forgeneration of a residual attracting force and the effect of the presentinvention will be described with reference to FIGS. 22 to 25. FIG. 22shows a simple equivalent circuit of an electrostatic chuck in which theratio between the areas of the actual attracting portions of twoelectrodes (for example, electrodes A and B) is set at 2.8, that is,electrode A side: electrode B side=2.8 (152.5 cm²):1(54 cm²). Theequivalent circuit of the electrostatic chuck being actuated to attracta wafer will be briefly described. In the equivalent circuit shown inFIG. 22, a parallel circuit including an electrostatic capacity Ca ofthe electrode A and the resistance Ra of a dielectric film formed on theelectrode A is connected in series to a parallel circuit including anelectrostatic capacity Cb of the electrode B and the resistance Rb of adielectric film formed on the electrode B through a resistance Rw(sufficiently smaller than each of Ra and Rb) of the wafer.

Letting Va, Vb be potential differences finally generated between theelectrodes A and B and the wafer when a voltage of 400 V is appliedbetween the electrodes A and B in such a state, the following equationsare given in a stable state.

    Va+Vb=400                                                  (1)

    Ra:Rb=Va:Vb                                                (2)

In the case of using a dielectric film made from a ceramic material,however, the volume resistivity thereof is changed depending on theapplied voltage, as shown in FIG. 23. Accordingly, letting V be theapplied voltage, the volume resistivity of the dielectric film of theelectrostatic chuck used for the present invention is expressed by thefollowing equation:

    volume resistivity=1×10(.sup.11.953-0.000764 V)      (3)

Since the resistance of the actual attracting portion of the dielectricfilm formed on each electrode can be calculated from the volumeresistivity of the dielectric film, the potential difference betweeneach electrode and the wafer can be obtained on the basis of theequations (1) to (3). In this example, the potential difference Vabetween the electrode A and the wafer is 126 V, and the potentialdifference Vb between the electrode B and the wafer is 274 V.Incidentally, as for the dielectric film, the electrostatic capacity isobtained by dividing the product of the dielectric constant and the areaby the thickness. Here, assuming that the relative dielectric constantof the ceramic forming the dielectric film is 5, the electrostaticcapacity of the dielectric film is determined. Thus, the amount of anelectric charge stored on the dielectric film can be calculated on thebasis of the electrostatic capacity of the dielectric film thusdetermined and the potential difference on the dielectric film obtainedusing the equations (1) to (3). In actual operation, however, thereexists a space represented by a surface roughness between the wafer andthe dielectric film. Such a space may be regarded as substantially avacuum space as in a vacuum chamber even if there exists a heat transfergas. Now, in this example, assuming that the space distance is about 3μm and the thickness of the dielectric film is 300 μm, the spacedistance is one-hundredth of the thickness of the dielectric film. As aresult, even if the dielectric constant of the space is one-fifth ofthat of the dielectric film, the electrostatic capacity of the spacebecomes about 20 times that of the dielectric film. For this reason, theelectrostatic capacity of the space is used in place of that of thedielectric film for the above-described calculation. The results thuscalculated are summarized as follows: namely, the electrode A has anarea of 152.5 cm², a capacity of 46 nF, a potential difference with thewafer of 126 V, and an amount of electric charge of 5.8×10⁻⁶ coulomb C!;and the electrode B has an area of 54 cm2, capacity of 16 nF, potentialdifference with wafer of 274 V and an amount of electric charge of4.4×10⁻⁶ coulomb C!. From these results, it becomes apparent that thereis a difference between the amounts of the electric charges stored onthe actual attracting portions on the electrodes A and B.

FIGS. 24(a) to 24(c) are typical circuit diagrams showing changes in theamounts of the electric charges stored in capacitance components when aDC power supply is turned off from an attracting state. In theattracting state shown in FIG. 24(a), electric charges are unequallystored on the dielectric film formed on electrodes A and B in largeamounts. When the application of a DC voltage is stopped, the electriccharge stored on the dielectric film formed on the electrode B isquickly eliminated by way of circuits 1 and 2 because the resistance ofthe wafer is sufficiently smaller than the resistance of the dielectricfilm, as shown in FIG. 24(b). Besides, the electric charge remaining onthe dielectric film on the electrode A is eliminated by way of thecircuit 3 or 4 as shown in FIG. 24(c); however, the elimination of theelectric charge takes a long time because the circuit 3 or 4 has a largevalue of resistance Ra or Rb, that is, it has a large discharge timeconstant. Such a remaining electric charge becomes the cause forgeneration of a residual attracting force.

On the other hand, in the case where the ratio between areas of theactual attracting portions of a dielectric film on the two electrodes is1:1, as in the embodiment of the present invention, since the attractingportions of the dielectric film on the two electrodes have the sameresistance and also have the same potential difference with the wafer,the electric charges stored on the attracting portions are equal to eachother. Accordingly, when the application of a DC voltage is stopped, theelectric charges on the attracting portions are eliminated only by wayof the circuits 1 and 2 shown in FIG. 24(b), so that it takes a shorttime to eliminate the electric charges and thereby no residualattracting force remains on the attracting portions.

FIG. 25 is a graph showing residual attracting force generating stateswhen the ratio between areas of the attracting portions of a dielectricfilm formed on two electrodes is changed. In this figure, the abscissaindicates the time elapsed after cutting off the DC power supply, andthe ordinate indicates the residual attracting force. From the resultsshown in FIG. 25, it is revealed that the residual attracting force isnot generated when the area ratio between the actual attracting portionson the two electrodes is 1:1; however, it becomes larger with increasein the area ratio.

Accordingly, in an electrostatic chuck having a configuration whereinthe ratio between the areas of the wafer attracting portions of adielectric film formed on two electrodes is 1:1, as in this example,little residual attracting force is generated and it takes a short timeto eliminate the electric charges stored on the wafer attractingportions. A sample processing apparatus including such an electrostaticchuck is also advantageous in improving the throughput of the apparatusand preventing a wafer from being broken when the wafer is pushed up bylift pins or the like after termination of the processing of the wafer.

Hereinafter, a first embodiment of the present invention will bedescribed with reference to FIGS. 1 to 8.

FIG. 1 shows one example of a sample processing apparatus using anelectrostatic chuck, representing a first embodiment of the presentinvention. The sample processing apparatus is represented by an etchingapparatus, a processing apparatus using plasma, such as a film formationapparatus, or a vacuum processing apparatus not using plasma, such as anion injection apparatus. In this embodiment, description will be made byway of example with reference to a plasma processing apparatus.

Referring to FIG. 1, there is shown a vacuum chamber 1 to which a gassupply unit 2 and an evacuation unit 3 are connected. The vacuum chamber1 is provided with a plasma generating unit 4 for generating a plasma 5in the vacuum chamber 1. In the vacuum chamber 1, there is provided asample stage on which a sample to be processed by the plasma 5, forexample, a substrate 9, such as a semiconductor substrate (or wafer) ora liquid crystal substrate, is mounted. The sample stage comprises anelectrostatic chuck 10.

The electrostatic chuck 10 is composed of an inner electrode 11, a ringelectrode 12, an insulating film 13, and an insulating film (ordielectric film) 14 for electrostatic attraction. A coolant passage 21is formed in the electrode 11. A ring-shaped recess in which theelectrode 12 is to be formed, is formed in the top surface of theelectrode 11. The electrode 12 is formed into a ring-shape. Theelectrode 11 is made from a conductive material, such as an aluminumalloy. In the recess formed in the top surface of the electrode 11,there is provided the electrode 12 supported on the insulating film 13.The insulating film 13 is formed from alumina by thermal spraying, andthe electrode 12 is formed from tungsten by thermal spraying. Theinsulating film 13 is interposed between the electrodes 11 and 12 fordirectly insulating the electrodes 11 and 12 from each other. On thesurfaces of the electrodes 11 and 12, there is formed the insulatingfilm 14 for electrostatic attraction. The insulating film 14 is formedfrom alumina by thermal spraying. In addition, the insulating film 13 ismade from a material having a resistance higher than that of theinsulating film 14 for electrostatic attraction. This is because anelectric circuit for electrostatic attraction is formed through theinsulating film 14.

A lead wire 18 is connected to the inner electrode 11 for applying avoltage thereto, and a lead wire 16 is connected to the ring electrode12 for applying a voltage thereto. The lead wire 16 is connected to thering electrode 12 by way of a through-hole formed of an insulatingsleeve 15 in the inner electrode 11. The lead wire 16 is electricallyinsulated from the inner electrode 11 by the insulating sleeve 15. Thelead wires 16 and 18 are connected through low pass filters 19a and 19bto power supplies 8a and 8b for electrostatic attraction, respectively.A negative voltage is applied from the DC power supply 8a to the ringelectrode 12, and a positive voltage, which has the same absolute valueas that of the negative voltage applied to the ring electrode 12, isapplied from the DC power supply 8b to the inner electrode 11. Theelectrodes 11 and 12 can be grounded by turning terminals 82a and 82b toterminals 83a and 83b using switches 84a and 84b, respectively. Theinner electrode 11 and the ring electrode 12 are electrically insulatedfrom the substrate 9 by the insulating film 14 for electrostaticattraction. Accordingly, by applying positive and negative voltages fromthe power supplies 8a and 8b to the inner electrode 11 and the ringelectrode 12, respectively, a DC circuit is formed through the substrate9 and thereby electric charges are stored, so that the substrate 9 canbe electrostatically attracted to the top surfaces of the electrodes 11and 12.

For connection of the lead wire 16 to the ring electrode 12, as shown inFIG. 3, a flange is provided on the upper portion of the insulatingsleeve 15, and an electrode core 161 is provided in the upper space ofthe flange while a socket 162 is provided in the lower space of theflange. The socket 162 is fixed to the flange by being screwed in theelectrode core 161. The lead wire 16 is inserted in the socket 162 to bethus connected thereto. The ring electrode 12 is formed by thermalspraying in a state wherein the electrode core 161 is fixed in theflange of the insulating sleeve 15. Thus, the electrode core 161 can beeasily connected to the ring electrode 12. Here, the electrode core 161is made from the same material, i.e. tungsten, as that of the ringelectrode 12 so as to ensure proper connection with the ring electrode12. In addition, the lead wire 18 can be easily connected to the innerelectrode 11 by a method wherein a female thread portion (not shown) isformed in the inner electrode 11 and a male thread portion is formed atthe leading end portion of the lead wire 18, whereby the male threadportion of the lead wire 18 is screwed in the female thread portion ofthe inner electrode 11.

A central portion of the inner electrode 11 is pierced to form athrough-hole 20 in which an insulating sleeve is provided. Thethrough-hole 20 is used for introducing a heat transfer gas from asource 6 to the back surface of the substrate electrostaticallyattracted on the electrostatic chuck 10. Here, the insulating film 14for electrostatic attraction is formed by thermal spraying and isfinally finished by polishing into a flat shape having a specificthickness. The use of the insulating film 14 formed by thermal sprayingallows a groove to be easily formed in the surface of the innerelectrode 11 or ring electrode 12 after formation of the insulating film14 by previously machining the surface of the electrode 11 or 12 to forma recess (not shown). This facilitates an electrode design to provide agas dispersion groove in the surface of the electrode.

The gas dispersion groove (or gas groove) is provided in the surface ofthe electrode for supplying a heat transfer gas (for example, heliumgas) to the back surface of a substrate to be processed therebycontrolling the temperature of the substrate or adjusting a heattransfer characteristic for making the substrate temperaturedistribution more uniform. Here, as shown in FIG. 2, there is provided agas dispersion groove composed of a plurality of circumferentiallyextending groove components partially connected to each other in theradial direction. The gas dispersion groove has a depth of 0.3 mm.

An attracting surface of the insulating film 14, which has no dispersiongroove and is brought in direct-contact with the substrate 9, hasattracting surfaces A1 to A4, B1 to B4, and D corresponding to the innerelectrode 11; and attracting surfaces C1 to C4 corresponding to the ringelectrode 12. The area relationship of these attracting surfaces is setsuch that the total area of the attracting surfaces A1 to A4, B1 to B4,and D is equal to the total area of the attracting surfaces C1 to C4.

The insulating film 13, ring electrode 12, and insulating film 14 areformed by thermal spraying to thicknesses of 0.3 mm, 0.1 mm, and 0.4 mm,respectively. The contact surface of the insulating film 14 with thesubstrate 9 is polished to a thickness of 0.3 mn. The total thickness ofthe films formed on the inner electrode 11 by thermal spraying is 0.8 mmor less. That is, while the total thickness of the films formed on theinner electrode 11 by thermal spraying is maximized at a portion of thering electrode 12, even such a maximized total thickness is very thin,such as 0.8 mm. Accordingly, the presence of the insulating films exertsonly a negligible effect on application of a radio frequency voltage tothe entire inner electrode 11, and thereby it does not affect processingof the substrate 9.

The electrostatic chuck 10 is mounted on the bottom surface of thevacuum chamber 1 through a grounding plate 24. The inner electrode 11 ismounted on the grounding plate 24 through an insulating plate 23. Toprevent leakage of the heat transfer gas from the through-hole 20provided in the central portion of the inner electrode 11 when the heattransfer gas is supplied into the through-hole 20, contact portions withthe through-hole 20 are sealed. The inner electrode 11, insulating plate23, and grounding plate 24 are fastened to each other with bolts (notshown).

A cover 22 is provided around the outer surface of the inner electrode11 in such a manner that the outer peripheral portion thereof issmoothly tilted inward in the upward direction. When ions in a plasmaare irradiated from top to bottom, the cover 22 formed into such a shapehas no shady portion, so that reaction products, if deposited on thecover 22 during plasma etching, can be easily removed by exposing thecover 22 to a cleaning plasma. As a result, it is possible to easilyreduce contaminants.

A radio frequency power supply 7, as well as the power supply 8b forelectrostatic attraction, is connected to the inner electrode 11. Theradio frequency power supply 7 is provided for applying a radiofrequency bias voltage to the inner electrode 11. To prevent occurrenceof abnormal discharge between the inner electrode 11 and the groundingplate 24, the diameter of the insulating plate 23 is set to be largerthan that of the inner electrode 11 and to be smaller than that of thegrounding plate 24 for preventing the inner electrode 11 from directlyfacing the grounding plate 24. With this configuration, it is notrequired to provide a separate insulating member around the outersurface of the inner electrode 11. That is, the cover 22 can serve assuch an insulating member.

The temperature of the substrate 9 shown in FIG. 1 is controlled on thebasis of the temperature of a coolant flowing in the coolant passage 21provided in the inner electrode 11. Specifically, the temperature of theinner electrode 11 is controlled on the basis of the temperature of thecoolant, and then the temperature of the substrate 9 is controlled byheat transfer from the cooled inner electrode 11 through the insulatingfilm 14 and by the heat transfer gas supplied on the back surface of thesubstrate 9. In this embodiment, although the coolant passage 21 isprovided only in the inner electrode 11, the temperature of the ringelectrode 12 is also controlled by heat transfer from the innerelectrode 11 through the thin insulating film 13. Accordingly, thecoolant is not required to be supplied to the ring electrode 12, thatis, it its sufficient for cooling the electrode 11 and 12 to provide thecoolant passage 21 only in the inner electrode 11, thereby simplifyingthe cooling mechanism.

In the plasma processing apparatus having the above configuration, anegative voltage is applied to the ring electrode 12, and a positivevoltage, which has the same absolute value as that of the voltageapplied to the ring electrode 12, is applied to the inner electrode 11.With such application of positive and negative voltages to theelectrodes 11 and 12, the electrode potentials shown in FIG. 4 areobtained.

FIG. 4 shows potentials of a substrate and electrodes of anelectrostatic chuck when the substrate electrostatically attracted onthe chuck is exposed to a plasma. In the state shown in the figure, theplasma is generated by a power supplying means provided separately frompower supplies for applying voltages to the electrodes of theelectrostatic chuck. Specifically, in the example shown in FIG. 4, thereare shown potentials of the substrate 9, ring electrode 12, and innerelectrode 11 in a state wherein the substrate 9 is electrostaticallyattracted on the electrostatic chuck in a condition where a voltage of-250 V is applied to the ring electrode 12 and a voltage of +250 V isapplied to the inner electrode 11. In the electrostatic chuck thusconnected to the DC power supplies, the potential of the substrate 9(wafer) attracted on the chuck is 0 V. Accordingly, even if thepotential of the wafer is changed from 0 V to about -20 V by generationof a plasma, there occurs only a small change in potential differencebetween the wafer and each electrode. As a result, the change inelectric charge stored between the wafer and each electrode is alsosmall.

In the electrostatic chuck according to this embodiment, in which theattracting areas corresponding to the positive and negative electrodesare equal to each other and the power supplies are connected to thechuck such that DC voltages different in polarity and equal in absolutevalue are applied to the electrodes for electrostatic attraction, theresidual attracting force of the chuck in a state where only a plasma isgenerated is very small. As a result, the effect of the residualattracting force on removal of the wafer from the electrostatic chuck issubstantially negligible. Also, as the DC voltages are continued to beapplied from the power supplies for electrostatic attraction afterplasma extinction, the potential of the wafer is returned to theoriginal state with no generation of plasma. Thus, the potentialdifference between the wafer and each electrode becomes zero. As aresult, in the electrostatic chuck having attracting areas equal to eachother, the amounts of electric charges stored on the attracting areasare equal to each other in accordance with the principle described withreference to FIGS. 22 to 25, and so electric charges remaining on theattracting areas corresponding to the electrodes are eliminated when theDC power supplies are turned off. In ether words, the electrostaticchuck in this embodiment has an effect of eliminating generation of aresidual attracting force.

On the other hand, in some cases, to promote processing of a substrate,a radio frequency voltage is applied to a sample stage for generating abias potential (generally, about -300 V or less) at the substrate. Inthis case, as shown in FIG. 4, the potential differences between thesubstrate and the electrodes are changed, so that there occurs a largedifference between the amounts of electric charges stored on theattracting areas corresponding to the electrodes. Even in this case,however, a residual attracting force can be reduced to zero by applyingDC voltages to the electrodes for a specific time after plasmaextinction. Further, by stopping application of the radio frequencyvoltage during generation of plasma and then maintaining generation ofplasma for a specific time, the potential difference between the waferand each electrode can be reduced to that in the above-described plasmageneration state without application of the radio frequency voltage,that is, to a value within the negligible range of about -20 V or less.In this state in which the potential difference between the wafer andeach electrode is about 40 V, the attracting force is significantlysmall, and accordingly, when being pushed up using lift pins, thesubstrate is not cracked. Consequently, with elimination of a residualattracting force in the case where a radio frequency voltage is appliedto a sample stage to promote processing a wafer, the residual attractingforce can be effectively eliminated by adjusting the time from thestopping of supply of the radio frequency voltage to plasma extinctionand a time from plasma extinction to the stopping of supply of the DCvoltages for electrostatic attraction.

In addition, when a radio frequency voltage is applied as shown in FIG.4, the potential difference between the wafer and the inner electrode(positive electrode) is made larger and the potential difference betweenthe wafer and the ring electrode (negative electrode) is made smaller.In the electrode configuration of this embodiment, since the innerelectrode forming the attracting components positioned at the outerperipheral portion and the central portion of the electrostatic chuck isprovided as a positive electrode, the electrostatic chuck can stronglyhold the outer peripheral portion and the central portion of the wafer.This is effective to more preferably suppress leakage of the heattransfer gas from the outer peripheral portion of the wafer duringplasma processing. Further, such an electrode configuration is effectiveto cool the central portion of the wafer more strongly because thecentral portion of the wafer is strongly attracted on the chuck. In thecase where the central portion of the wafer is not intended to bestrongly cooled, the heat transfer efficiency at the gas groove portioncorresponding to the central portion of the wafer is improved byenlarging the area and depth of the gas groove portion. In this case,with respect to the attracting portion corresponding to the ringelectrode 12, the area thereof is made smaller and also the depth of thegas groove portion is made smaller, as compared with the attractingportion of the inner electrode 11.

Next, a procedure of attracting a substrate, starting plasma processing,terminating plasma processing, and eliminating electric charges in thesubstrate will be described in this order with reference to a time chartshown in FIG. 5. First, a substrate is carried into the vacuum chamber 1by a carrier (not shown). After the substrate is placed on theelectrostatic chuck 10, a DC voltage is applied between the positive andnegative electrodes 11 and 12 for attracting the substrate, and then aheat transfer gas is introduced in the gas groove provided in thesurface of the insulating film (dielectric film) 14. At this time, aprocessing gas for processing the substrate has been already introducedinto the vacuum chamber 1 by the gas supply unit 2 and kept at aspecific pressure. Then, energy (for example, microwave electric field,radio frequency electric field or the like) for generation of a plasmais introduced into the vacuum chamber 1 by the plasma generating unit 4.A plasma is thus generated in the vacuum chamber 1. Next, a radiofrequency voltage for generating a bias voltage at the substrate isapplied. It is to be noted that the necessity of applying a radiofrequency voltage is dependent on the process used, and that in the caseof applying a radio frequency voltage, application and stopping of theradio frequency voltage is performed during stable generation of plasmafor matching of impedance. The plasma extinction is performed bystopping introduction of the energy for generating the plasma,simultaneously with termination of the plasma processing the wafer. Inaddition, supply of the radio frequency voltage is stopped before plasmaextinction. Here, plasma extinction is performed after an elapse of fourseconds following stopping of the radio frequency voltage. Thiseliminates, as described above, an unbalance between electric chargesstored on attracting portions of the insulating film (dielectric film)formed on the electrodes 11 and 12 during plasma processing. Aftertermination of the processing of the substrate, the supply of the heattransfer gas, which becomes unnecessary, is stopped and the heattransfer gas remaining in a dispersion groove and a gas supply passage(both, not shown) is exhausted. Then, the wafer is removed from theelectrostatic chuck and carried; however, prior to removal of the wafer,the processing gas, which is usually composed of a harmful gas, must besufficiently exhausted. In this embodiment, the exhausting of theprocessing gas is performed for about ten seconds, and elimination ofelectric charges (residual attracting force) stored on the electrostaticchuck is terminated during the time required for the exhausting of theprocessing gas. Specifically, introduction of the heat transfer gas andthe processing gas is stopped after an elapse of one second followingplasma extinction, and an exhausting of the heat transfer gas remainingin the dispersion groove is performed for 0.5 second. After that, thesupply of the DC voltage for electrostatic attraction is terminatedafter an elapse of three seconds following plasma extinction. Thisoperation of maintaining the supply of the DC voltage for three secondsafter plasma extinction, as described above, reduces the unbalancebetween the electric charges stored on the attracting portions of theinsulating film (dielectric film) on the electrodes 11 and 12, exceptfor the unbalance between the electric charges which have beeneliminated by maintaining generation of the plasma after stopping of theradio frequency voltage. Thus, since the amounts of the electric chargeson both the electrodes are balanced, the electric charges polarized onboth the electrodes are quickly extinguished for about two or threeseconds after stopping the DC voltage. Consequently, the substrate canbe carried directly after termination of the exhausting of theprocessing gas. After carrying the substrate from the vacuum chamber, anew substrate to be processed is carried into and processed in thevacuum chamber in the same manner as described above. Such a cycle willbe repeated. And, if there is no substrate to be processed, theprocessing is terminated.

In this way, the final elimination of electric charges stored on theelectrostatic chuck can be terminated during the time required forexhausting the processing gas, and consequently, it is not required toset a special time required for eliminating electric charges stored onthe chuck. This is effective to improve the working ratio of theapparatus.

While in the time chart shown in FIG. 5, the time from the stopping ofsupply of a radio frequency voltage to plasma extinction is set at fourseconds, it may be suitably set depending on the time required foreliminating a residual attracting force (or eliminating an imbalancebetween electric charges on both electrodes) after stopping of theplasma. FIG. 6 shows a relationship between a residual attracting forceand a time from the stopping of supply of a radio frequency voltage toplasma extinction. From data shown in FIG. 6, it is revealed that theresidual attracting force is not reduced so much in the case whereplasma extinction is performed until an elapse of about three secondsfollowing the stopping of supply of the radio frequency voltage; it isreduced to about a half the original value in the case where plasmaextinction is performed after an elapse of about four seconds followingthe stopping of supply of the radio frequency voltage; and it is reducedto a low and substantially constant value in the case where plasmaextinction is performed after elapse of about five seconds following thestopping of supply of the radio frequency voltage. The above lowresidual attracting force after elapse of five seconds is due to thepotential differences generated in the case where only the plasma isapplied without applying the radio frequency voltage. Accordingly, asdescribed above, there is no problem even when the substrate is removedfrom the electrostatic chuck in the state where a low residualattracting force remains.

Next, a manner of removing a substrate from the electrostatic chuck willbe described with reference to FIGS. 7 and 8. Insulating sleeves 34 areprovided in the inner electrode 11 at a plurality of positions. A liftpin 30 for removing the substrate 9 from the mounting surface of theelectrostatic chuck is provided in each of the insulating sleeves 34 insuch a manner as to pass through the insulating sleeve 34. A steppingmotor 32 is mounted on the lower portions of the lift pins 30 through aload cell 31. A signal from the load cell 31 is inputted into a controlunit 33. The control unit 33 outputs a signal for controlling thestepping motor 32. A cover 22 is provided in such a manner as tosurround the outer peripheral portion of the electrode 11 and the outerperipheral portion of the substrate 9 in a state in which the substrate9 is mounted on the insulating film 14 of the electrostatic chuck. Here,a gap between the outer peripheral end surface of the substrate 9 andthe cover 22 is within an allowable range of about 1 mm or less. Theallowable range of the above gap is set to allow the substrate 9 to becarried to a carrier (not shown) with no problem even when the substrate9 is offset on the lift pins 30 when removed from the electrostaticchuck using the lift pins 30. With this configuration of the samplestage, even when a residual attracting force remains somewhat, thesubstrate 9 can be forcibly removed from the electrostatic chuck by thelift pins 30. Specifically, even in the case where the substrate 9 issubjected to a force greater than the residual attracting force when thelift pins are lifted, and tends to jump, the position of the substrate 9is held by the cover 22. This makes it possible to remove the substrate9 with safety even in the case where the residual attracting force isnot perfectly eliminated.

Upon removal of the substrate 9, as shown in FIG. 8, as the lift pins 30are lifted, the load observed by the load cell 31 is increased at aspecific ratio just as in the case where a spring load is applied by acomponent such as bellows. Here, when the lift pins 30 are brought incontact with the back surface of the substrate 9 attracted on the chuckwith a residual attracting force, the load cell 31 additionally detectsa load due to the residual attracting force. Such an additional load dueto the residual attracting force is shown as a locally projecting loadappearing in FIG. 8. In this embodiment, to prevent the substrate 9 frombeing cracked or abnormally jumping due to the residual attracting forcewhen the substrate 9 is forcibly pushed up by the lift pins 30, thepush-up force of the lift pins 30 is set at an allowable value. Theallowable push-up force is stored in the control unit 33, and the liftpins 30 are lifted by the stepping motor 32 on the basis of theallowable push-up force. Specifically, when the load detected using theload cell 31 exceeds the allowable push-up force by lifting the liftpins 30 after the lift pins 30 are brought in contact with the substrate9, the control unit 33 operates the stepping motor 32 so as to retardthe lifting rate of the lift pins 30 or to stop the lifting of the liftpins 30. This prevents the substrate 9 from being damaged or erroneouslycarried.

Accordingly, using the above control for removing the substrate 9, itbecomes possible to start removal of the substrate 9 after plasmaextinction and remove the substrate 9 directly after stopping the supplyof the DC voltage for electrostatic attraction, and to improve thethroughput in carrying the sample.

As described above, with the bipolar type electrostatic chuck formingthe first embodiment, having a gas groove in a sample mounting surface,the amounts of electric charges stored on attracting portionscorresponding to the positive and negative electrodes directly beforestopping the supply of DC voltage for electrostatic attraction are setto be equal to each other, and accordingly, when the supply of the DCvoltage is stopped, the electric charges equally stored on both theelectrodes are eliminated, that is, no electric charges remain on eitherof the electrodes. As a result, it is not required to provide anyspecial means for eliminating electric charges stored on the chuck afterstopping the supply of the DC voltage. This improves the throughput incarrying the sample.

Further, according to the first embodiment, the same insulating film forelectrostatic attraction is formed on the inner electrode and the ringelectrode, and the areas of the attracting portions corresponding to thepositive and negative electrodes excluding the gas groove portion areequal to each other, and accordingly, the amounts of electric chargesstored on attracting portions corresponding to the positive and negativeelectrodes directly before stopping the supply of a DC voltage forelectrostatic attraction are set to be equal to each other, so that whenthe supply of the DC voltage is stopped, no electric charges remain oneither of the electrodes. As a result, it is not required to provide aspecial means for eliminating electric-charges stored on the chuck afterstopping the supply of the DC voltage. This improves the throughput incarrying the sample.

In this way, in the electrostatic chuck having two electrodes asrepresented by the first embodiment, since the ratio between the areasof the wafer attracting portions of the dielectric film positioned onthe two electrodes is specified at 1:1, little residual attracting forceis generated, with a result that the time required for eliminating theelectric charges stored on the chuck can be shortened. Accordingly, in asample processing apparatus including the electrostatic chuck accordingto the first embodiment, it becomes possible to improve the throughputof the apparatus, because the time required for eliminating the electriccharges is short, and to prevent the breakage of the wafer upon pushingup the wafer using pushers or the like after termination of theprocessing of the wafer, because little residual attracting force isgenerated.

In the electrostatic chuck forming the first embodiment, further, sincea pair of the inner electrode and the ring electrode are concentricallydisposed, a processing condition is equally applied to the entiresubstrate in such a manner as to be symmetric around the center of thesubstrate, so that it is possible to equally process the substrate.

Additionally, in the electrostatic chuck according to the firstembodiment, the residual attracting force is eliminated after stoppingof supply of DC voltage, and accordingly, even after removal of thesubstrate from the electrostatic chuck, contaminants having electriccharges are suppressed from adhering on the substrate mounting surfaceas compared with the case where the residual attracting force exists,with a result that there is no fear that contaminants adhere on the backsurface of a new substrate.

Although a positive and a negative voltage having the same potential areapplied to the inner electrode 11 and the ring electrode 12 in the firstembodiment, the values of the voltages applied from the DC powersupplies 81a and 81b to the electrodes 11 and 12 may be varied duringplasma processing such that the attracting voltages of both theelectrodes are equal to each other based on the bias voltage. With thisadjustment of the voltages, since the electrostatically attracting areascorresponding to both the electrodes are equal to each other, theelectrostatically attracting forces thereof become equal to each otherduring plasma processing, with a result that it is possible to preventextreme unevenness of a temperature distribution of the sample surface.

With respect to the arrangement of the pair of electrodes, the firstembodiment has been described by way of example with reference to thearrangement shown in FIG. 9(a) in which the electrode 12 is disposedslightly inward from the outer peripheral portion of the electrode 11;however, as shown in FIG. 9(b), the electrode 12 may be disposed at theouter peripheral portion of the electrode 11, or it may be disposed at acentral portion of the electrode 11, as shown in FIG. 9(c).

The arrangement shown in FIG. 9(b) is advantageous in that a recess inwhich a ring electrode 12a is to be provided can be easily machined,which contributes to a reduction in cost. Further, since one end of thering electrode 12a is in a stress relief state, the ring electrode 12ais not damaged, for example cracked, when it undergoes a thermal cycle.According to the arrangement shown in FIG. 9(c), the outer side of anelectrode 12b can be easily machined upon formation of a gas groove; andwhile the plasma processing apparatus is generally difficult withrespect to effecting temperature control of the outer peripheralportion, since the outer peripheral portion of the electrode 12b has ahigh degree of freedom of design for the gas groove, the temperaturecontrol for the outer peripheral portion of the electrode 12b can beeasily performed.

With respect to connection of DC power supplies to a pair of electrodes,in the first embodiment, the DC power supplies are connected to theelectrodes such that a positive potential is applied to the innerelectrode 11 and a negative potential is applied to the ring electrode12; however, there may be adopted a connection as shown in FIG. 10.According to the connection shown in FIG. 10, the inner electrode 11 isgrounded, and the power supply 8a for electrostatic attraction isconnected such that a negative potential is applied to the ringelectrode 12. FIG. 11 shows potentials of a wafer and the electrodeswhen the wafer is electrostatically attracted and held on the chuckshown in FIG. 10 and is exposed to a plasma generated by the plasmagenerating unit. If -500 V is applied to the ring electrode 12, thepotential of the wafer attracted on the chuck becomes -250 V and thepotential of the inner electrode becomes 0 V. Accordingly, the potentialdifference (250 V) between the wafer and the ring electrode 12 is equalto the potential difference (250 V) between the wafer and the innerelectrode 11. As a result, the attracting forces at both the electrodeportions are also equal to each other. Then, when the wafer is exposedto the plasma, a bias potential of about -20 V is generated at thewafer, so that the potential difference between the wafer and eachelectrode is changed. Here, the potential difference between the waferand the inner electrode 11 is changed from 250 V to 20 V, and thepotential difference between the wafer and the ring electrode 12 ischanged from 250 V to 480 V. As a result, the attracting force at theinner electrode portion is decreased, while the attracting force at thering electrode portion is increased. Thus, a cooling gas flowing on theback surface of the wafer is sufficiently sealed in the vicinity of theouter peripheral portion of the wafer, thereby desirably preventingleakage of the heat transfer gas. Further, while in plasma processing,the temperature at the outer portion of the wafer tends to be increasedand thereby the outer portion of the wafer must be more strongly cooled,according to this embodiment, and so the temperature distribution of thewafer during plasma processing can be effectively equalized because theattracting force at the ring electrode 12 portion is increased.

FIG. 12 shows another type of connection in which the power supply 8afor electrostatic attraction is connected with the ring electrode 12 andthe inner electrode 11, which is in a floating state with respect to theground potential, and the ring electrode 12 is applied with a voltagehaving a potential lower than that of the inner electrode 11. Inaddition, each electrode can be have the same potential applied theretoby operation of a switch 84a. FIG. 13 shows potentials of a wafer, thering electrode, and the inner electrode when the wafer iselectrostatically attracted and held on the electrostatic chuck in whicha potential difference of 500 V is generated between the ring electrode12 and the inner electrode 11 in accordance within the connection shownin FIG. 12. In the electrostatic chuck having such a configuration, thepotential of the wafer becomes an intermediate value between potentialsof the ring electrode 12 and the inner electrode 11, and the potentialsof both the electrodes 11 and 12 become equal to each other. In the casewhere the wafer is exposed to the plasma in such a state and also aradio frequency voltage is applied to the wafer to generate a biaspotential, since the voltages applied to the ring electrode 12 and theinner electrode 11 are in the floating state with respect to groundpotential, both the bias potential and the wafer potential are changed,with a result that the potential difference between the wafer and eachelectrode is not changed. Accordingly, the amount of the electric chargestored on the actual attracting portion of the dielectric film positionon each electrode is not changed, so that the attracting forcedistribution is also not changed. As a result, there can be obtained aneffect in which little residual attracting force is generated becausethe attracting force is not changed. Although in the example shown inFIG. 12, the ring electrode 12 is applied with a voltage having apotential lower than that of the inner electrode 11, it may be appliedwith a voltage having a potential higher than that of the innerelectrode 11. Even in this case, there can be obtained the same effect.

Even in the connection shown in FIG. 10, in some cases, a radiofrequency voltage is applied to a wafer for generating a bias potential(usually, about -300 V or less) at the wafer thereby promotingprocessing of the wafer. In this case, potential differences between thewafer and the electrodes are changed, and thereby there occurs adifference between the amounts of the electric charges stored on theelectrode portions. To reduce such an unbalance between the amounts ofthe electric charges stored on the electrode portions (a residualattracting force), as described above, generation of the plasma may bemaintained for a specific time after stopping the supply of the radiofrequency voltage, or the DC voltage may be continued to be applied fora specific time after plasma extinction.

In the electrostatic chuck in which the DC power supply is connected asshown in FIGS. 10 and 12, by specifying the amounts of the electriccharges stored on the actual attracting portions of the dielectric filmpositioned on the positive and negative electrodes to be substantiallyequal to each other, the stored electric charges are smoothly eliminatedand little residual attracting force is generated. In the case where avery large attracting force is required, however, a large DC voltage itsrequired to be applied between the positive and negative electrodes. Inthis case, the amounts of electric charges stored on the dielectric filmare naturally increased, so that it takes several seconds or severaltens of seconds to eliminate the stored electric charges. To limit theincreased time required for eliminating the stored electric charges, avoltage having a polarity reversed to that applied during electrostaticattraction may be applied between the positive and negative electrodes.Thus, there can be provided an electrostatic chuck and a sampleprocessing apparatus which are capable of shortening the time requiredfor eliminating the stored electric charges.

Next, a second embodiment of the present invention will be describedwith reference to FIGS. 14 to 17.

Referring to FIG. 14, there is shown a basic structure of anelectrostatic chuck forming the second embodiment of the presentinvention. A dielectric film 35 is fixed on an aluminum block 34 throughan adhesive layer 36. The dielectric film 35 is formed of an aluminasintered body. Two electrodes, a ring electrode 31 and an innerelectrode 32, are concentrically buried in the dielectric film 35. Eachof the two electrodes 31 and 32, made from tungsten, has a thickness ofabout 50 to 100 μm. A DC voltage is applied to each of the electrodes 31and 32 through a lead wire 38 perfectly sealed by an insulating resinlayer 43. The lead wire 38 is brazed with each electrode at a portion37. In this embodiment, ground potential is applied to the innerelectrode 32, and a DC power supply 40 is connected to the ringelectrode 31 through a switch 39. The ring electrode 31 can be connectedto the minus potential of the DC power supply 40 or the earth 41 byturning the switch 39. When a negative potential is applied to the ringelectrode 31 by the switch 39 in a state where a wafer is mounted on asurface 44 of the dielectric film 35, a potential difference isgenerated between the wafer and each electrode. This allows the wafer tobe electrostatically attracted and fixed on the contact surface 44. Whenthe ring electrode 31 is grounded by reversely turning the switch 39, anelectric charge stored between the wafer and each electrode iseliminated.

While the total thickness of the dielectric film 35 is 1 mm, thethickness of the dielectric film 35 on the electrodes 31 and 32 is 300μm. The surface roughness of the dielectric film is 3 μm. A gas groove46 having a depth of about 20 μm is formed in the surface 44 of thedielectric film 35, as shown in FIG. 14. The gas groove 46 is formed insuch a shape that a heat transfer gas for promoting cooling of a waferduring processing effectively flows over the entire back surface of thewafer. Into the gas groove 46, there is introduced a heat transfer gasfrom a heat transfer gas inlet 45 through an external pipe (not shown).The gas groove 46 is formed in a pattern capable of giving a desirabletemperature distribution to the wafer during processing. In thisembodiment, the ratio between an area of a wafer attracting portionpositioned on the ring electrode and an area of a wafer attractingportion positioned on the inner electrode is set at 1:1. Also, a ratiobetween an area of a gas groove portion on the ring electrode and anarea of a gas groove portion on the inner electrode is set at 1:1. Theelectrostatic chuck is provided with four lift pins 47 which areconcentrically arranged. The lift pin 47 is inserted in an insulatingsleeve 48 to be insulated from the electrodes 31 and 32 and the aluminumblock 34. The lift pins 47 are vertically driven by a lifting mechanism(not shown) such as an external motor, which are used for carrying awafer, the processing of which has been terminated.

With the electrostatic chuck having the above configuration, the depthof the gas groove is about one-tenth of the thickness of the dielectricfilm on the electrodes, and thereby the gas groove similarly generatesan electrostatically attracting force which is different from those ofthe actual attracting portions. FIGS. 16(a) to 16(c) show a relationshipbetween an electrostatic attracting force and the distance between awafer and a dielectric film. To examine a wafer attracting force, aspacer was provided on an electrostatic chuck, as shown in FIG. 16(a).The data are shown in FIGS. 16(b) and 16(c), wherein FIG. 16(b) showsthe case of using a mirror wafer and FIG. 16(c) shows the case of usinga wafer with a SiO2 film. From the data shown in FIGS. 16(b) and 16(c),it becomes apparent that in each case, little attracting force isgenerated when the thickness of the spacer, that is, the distancebetween the wafer and the dielectric film is more than 30 μm.Accordingly, for a gas groove having the depth smaller than 30 μm, anelectrostatic attracting force in the gas groove must be taken intoaccount. In particular, since an electrostatic attracting force iscertainly generated in a gas groove having the depth smaller than 20 μm,it must be examined.

In this embodiment, since the areas of the actual attracting portions onthe inner electrode and the ring electrode are equal to each other, andalso the areas of the gas groove portions on the inner electrode and thering electrode are equal to each other, the electric charges stored onthe actual attracting portions on the ring electrode and the innerelectrode are equal to each other, with a result that little residualattracting force is generated after stopping the supply of the DCvoltages, as in the first embodiment.

In the electrostatic chuck of the second embodiment, the dielectric filmis formed of an alumina sintered body. In general, a dielectric film ofan electrostatic chuck is made from a ceramic material. The ceramicmaterial, however, has a characteristic such that the resistivity isdependent on the applied voltage and the temperature, as describedabove. FIG. 17 shows a change in resistivity of the dielectric film usedfor the electrostatic chuck of the second embodiment depending on thetemperature of the dielectric film when a voltage of 200 V is appliedthereto. From this figure, it is revealed that the resistivity of thedielectric film at -50° C. is about 30 times that of the dielectric filmat 20° C. In the case of using a dielectric film having an excessivelylow resistivity, an electric charge is not stored between the surface ofthe dielectric film and the back surface of the wafer, and thereby anattracting force is not generated. On the other hand, in the case ofusing a dielectric film having an excessively high resistivity, thedischarge time constant of an electric charge stored on the surface ofthe dielectric film and the back surface of the wafer becomes larger,and thereby the time required for eliminating the stored electric chargeis made longer. In this case, a residual attracting force remains.

In a process requiring a fine processing with a good reproducibility,the temperature of an electrostatic chuck is generally controlled formanaging the temperature of a wafer during processing. However, sincethe wafer temperature range is changed depending on the kind of process,there is a possibility that the electrostatic chuck having a dielectricfilm largely depending on temperature change cannot be used for aprocess having a certain wafer temperature range. For example, in anetching apparatus, the wafer temperature is required to be controlled ina range of a low temperature, about -60° C. to about 100° C. In a filmformation apparatus using CVD or sputtering, the wafer temperature is ina range of 100° C. to a high temperature, about 700° C. In this case,the resistivity of a dielectric film is adjusted by addition of animpurity, such as a metal oxide, in a basic material of the dielectricfilm in order that it becomes a suitable value in a service temperaturerange.

An electrostatic chuck having the dielectric film thus adjusted is ableto quickly eliminate the electric charges on the chuck while ensuring asufficient attracting force in the entire range of service temperatures.Also, a sample processing apparatus using such an electrostatic chuck isable to improve the working ratio because one apparatus can carry outprocesses in a wide temperature range.

Additionally, in the case of using the electrostatic chuck for processeshaving different service temperature ranges, the attracting forcediffers between the service temperature ranges of the processes becausethe resistivity of the dielectric film is dependent on the temperature.The changed attracting force varies the thermal conductivity of anattracting portion, which possibly results in a change in the process.To cope with such an inconvenience, the applied voltage is changed togenerate a constant attracting force in a service temperature range onthe basis of the previously examined data on a change in resistivity ofthe dielectric film in the service temperature range.

A sample processing apparatus including the electrostatic chuck havingthe above configuration is allowed to usually process wafers with a goodproducibility.

In the above-described first and second embodiments, actual attractingareas corresponding to positive and negative electrodes are set to beequal to each other for making the amounts of positive and negativeelectric charges stored on an electrostatically attracting film(insulating film 14, dielectric film 35) equal to each other directlybefore stopping the supply of DC voltages for electrostatic attraction.In some cases, however, the above attracting areas cannot be set to beequal to each other. In these cases, there may be adopted the followingexpedient may be adopted.

For example, assuming that in FIG. 14, the area of the actual attractingportion on the inner electrode 32 is taken as 54 cm² and the area of theactual attracting portion on the ring electrode 31 is taken as 152.5cm², the area of the actual attracting portion on the ring electrode 31side is 2.8 times the area of the actual attracting portion on the innerelectrode 32 side. Accordingly, in order that amounts of electriccharges stored between the attracted wafer and the dielectric film onthe electrodes 31 and 32 are substantially equal to each other when aservice voltage of 400 V is applied, the surface roughness of thedielectric film on the inner electrode 32 is set at 3 μm and the surfaceroughness of the dielectric film on the ring electrode 31 is set at 3.9μm on the basis of the principle described with reference to FIGS. 22 to25. Here, potentials generated between the wafer and the electrodes 32and 31, and the electrostatic capacities of the dielectric film on theelectrodes 32 and 31, are calculated on the basis of the above-describedequations, as follows: namely, the potential between the wafer and theinner electrode 32 is 274 V and the potential between the wafer and thering electrode 31 is 126 V; and the electrostatic capacity of thedielectric film on the inner electrode 32 is 16 nF and the electrostaticcapacity of the dielectric film on the ring electrode 31 is 35 nF. Onthe basis of these conditions, the amounts of electric charges stored onthe dielectric film on the electrodes 32 and 31 are calculated asfollows: namely, the amount of the electric charge stored on thedielectric film on the inner electrode 32 is 4.4×10⁻⁶ coulomb, and theamount of the electric charge stored on the dielectric film on the ringelectrode 31 is 4.4×10⁻⁶ coulomb. This result shows that the amounts ofthe electric charges stored on the dielectric film on the electrodes 32and 31 are substantially equal to each other. Accordingly, when supplyof the DC voltages is stopped in such a state, generation of a residualattracting force is suppressed on the basis of the principle describedwith reference to FIGS. 22 to 25, with a result that the time requiredfor eliminating the stored electric charges is shortened.

Specifically, when an electrostatic chuck is designed such that theproduct of a ratio between electrostatic capacities of actual attractingportions of a dielectric film on respective electrodes and a ratiobetween resistances of the actual attracting portions of the dielectricfilm on the electrodes is set at approximately 1, that is, therelationship of Ca×Ra=Cb×Rb obtained from the relationship ofCa×Va=Cb×Vb is satisfied, the amounts of the electric charges stored onthe actual attracting portions of the dielectric film on the electrodesduring attraction of the wafer are equal to each other. Theelectrostatic chuck thus designed makes it possible to suppressgeneration of a residual attracting force.

In the above description, the attracting area on the inner electrode 32side is made small; however, in some processing conditions, theattracting area on the ring electrode 31 side may be made smaller. As aresult of an experimental examination of a relationship between anelectrostatic attracting force and a wafer temperature when a gas issupplied to the back surface of the wafer, it was found that the waferis more effectively cooled as the electrostatic attracting force becomeslarger. On the other hand, when the electrostatic capacities (Q=C×V) atrespective electrode portions are equal to each other, the attractingforce per unit area becomes larger as the electrostatic attracting areabecomes smaller. On the basis of the data, in the case where the outerportion of a sample is required to be more strongly cooled or heated inconsideration of a temperature distribution within a sample surface uponprocessing of the sample, the temperature distribution can be improvedby supplying a cooling gas on the back surface of the sample andstrongly attracting and holding the outer portion of the sample.Accordingly, in the case where the attracting areas are different, bysuitably setting the attracting areas on respective electrodes, thetemperature distribution within the sample surface can be adjusted.

Next, a third embodiment of the electrostatic chuck of the presentinvention will be described with reference to FIGS. 18(a) and 18(b). Inthis embodiment, a new dummy wafer 50 is placed on a dielectric film 53and is attracted by applying a voltage, which is larger than a voltageapplied in the actual processing, from a DC power supply 54. As aresult, contaminants adhering on the surface of the dielectric film, forexample, contaminants having a negative electric charge and which arenot allowed to be usually made repulsive by an negative electric chargegenerated during usual attraction of a wafer, are made repulsive by anegative electric charge larger than that generated during usualattraction of the wafer, and are transferred on the back surface of thewafer as shown in the enlarged view of FIG. 18(b). The dummy wafer isthen removed from the chuck in the same manner as that used for usualcarrying of the wafer. Thus, the contaminants adhering on the dielectricfilm can be removed. Although in this figure, only the contaminantshaving a negative electric charge are shown; however, actually,contaminants having a positive electric charge adhere on the surface ofthe dielectric film.

In the electrostatic chuck in which the above operation is periodicallyrepeated, it is possible to reduce the number of contaminants adheringon the back surface of a wafer to be processed, and to usually subjectwafers to a clean process. Accordingly, a processing apparatus includingthe electrostatic chuck in this embodiment is allowed to improve theyield of products. In addition, since the number of disassemblingoperations for cleaning contaminants stored in the apparatus can bereduced, the working ratio of the apparatus can be enhanced.

Although the manner of removing contaminants having a positive ornegative electric charge has been described in the third embodiment,another manner of removing contaminants having positive and negativeelectric charges will be described with reference to FIG. 19. In thiscase, the DC power supply shown in FIG. 18 is replaced with a DC powersupply capable of suitably switching polarity (positive or negative) ofan applied voltage. A new dummy wafer 50 is placed on the surface of thedielectric film 53, and a DC voltage having an absolute value largerthan that of a voltage applied for usual attraction of a wafer isapplied in such a manner as to be alternately changed in polarity, asshown in FIG. 19. With this operation, contaminants not allowed to beremoved by the operation shown in FIG. 18(a), that is, contaminantshaving a positive electric charge and which are electrostaticallyattracted on the dielectric film can be transferred on the dummy wafer,to be thus removed from the chuck. According to this embodiment,therefore, the dielectric film can be effectively cleaned.

In this embodiment, a new dummy wafer is used for removing contaminantson the dielectric film; however, it may be replaced with any member madefrom a conductive or semiconducting material in a clean state. However,it is best to avoid a member containing a material capable of causingheavy metal contamination.

In addition, although a DC voltage is applied in such a manner as to bealternately changed in polarity in this embodiment, the presentinvention is not limited thereto, and for example, the same effect canbe obtained by applying an AC voltage.

A fourth embodiment using the electrostatic chuck of the presentinvention will be described with reference to FIGS. 20 and 21. FIG. 20shows the configuration of a sample processing apparatus using theelectrostatic chuck of the present invention. The sample processingapparatus is composed of an atmospheric loader unit and a vacuumprocessing unit. The atmospheric loader unit has cassette mounting areason which a plurality of cassettes 61 can be mounted. The atmosphericloader unit also has an atmospheric carrying robot 62 for carryingwafers contained in each cassette 61 into the vacuum processing unit orfor returning wafers which have been processed in the vacuum processingunit to the cassette 61. The vacuum processing unit has a load lockchamber 63, unload lock chamber 64, and processing chambers A, B, C andD, which are indicated by reference numerals 70, 71, 72, and 73,respectively. These chambers are arranged around a vacuum carryingchamber 65 and are connected thereto. The load lock chamber 63 and theunload lock chamber 64 are positioned on the side of the atmosphericloader 60. A vacuum carrying robot 66 is provided in the vacuum chamber65. The vacuum carrying robot 66 includes an arm 67. The leading end ofthe arm 67 has a hand 68. The vacuum carrying robot 66 is actuated insuch a manner that the hand 68 is allowed to be inserted in each of thechambers 63, 64, 70, 71, 72 and 73. The hand 68 has wafer mountingsurfaces disposed on both ends thereof. The wafer mounting surfacedisposed at the leading end of the hand 68 is formed with theelectrostatic chuck shown in FIG. 21. The electrostatic chuck iscomposed of an outer electrode 681, an insulating film 682, an innerelectrode 683, and an insulating film 684 for electrostatic attraction.The outer electrode 681 disposed at the leading end of the hand 68 has,for example, three projections. A recess is formed in part of eachprojection, and the inner electrode 683 is provided in the recess. Aninsulating sleeve 685 is mounted in the recess of the outer electrode681 in such a manner as to pass through the outer electrode 681, and anelectrode core 686 is provided in the insulating sleeve 685. Aninsulating film 682 is formed on the surface of the recess by thermalspraying, and the inner electrode 683 is formed on the insulating film682 by thermal spraying. The inner electrode 683 can be easily connectedto the electrode core 686 by thermal spraying of the inner electrode683. On the top surfaces of the outer electrode 681 and the innerelectrode 683, there is formed an insulating film 684 by thermalspraying. A lead wine 689 is connected to the electrode core 686, and alead wire 688 is connected to the outer electrode 681. The lead wires688 and 689 are connected to a power supply for electrostatic attraction(not shown). An insulating cover 687 is provided on the bottom surfaceof the outer electrode 681. Here, in order to suppress adhesion ofcontaminants, the wafer contact surface of an electrostatic attractingportion formed on the projection is made as small as possible. Further,the areas of electrostatic attracting surfaces corresponding to theouter electrode 681 and the inner electrode 683 are equal to each other.

With a sample processing apparatus having the above configuration, awafer is taken out of the cassette 61 and is carried into the load lockchamber 63 by the atmospheric robot 62. The wafer thus transferred intothe load lock chamber 63 is then carried into a designated processingchamber (for example, processing chamber 71) by the vacuum carryingrobot 66. At this time, the hand 68 receives at the one end the wafer69, which has been processed in the processing chamber 71, is turned,and carries a non-processed wafer into the processing chamber 71. Thealready processed wafer held by the one end of the hand 68 is carriedinto the next processing chamber (for example, processing chamber 70) bythe vacuum carrying robot 66. On the other hand, the wafer to beprocessed in a different processing chamber (for example, processingchamber 72) is carried by a similar operation of the atmosphericcarrying robot 62 and the vacuum carrying robot 66.

Here, when the vacuum carrying robot 66 receives the wafer from the loadlock chamber 63 or each processing chamber, positive and negative DCvoltages having the same potential are applied to the outer electrode681 and the inner electrode 683, so that the amounts of electric chargesstored on the electrostatically attracting surfaces of the insulatingfilm positioned on the electrodes 681 and 683 are equal to each other.When the vacuum carrying robot 66 delivers the wafer into the unloadlock chamber 64 or each processing chamber, the supply of the DCvoltages applied to the outer electrode 681 and the inner electrode 683is stopped, so that the electric charges stored on the electrostaticattracting surfaces of the insulating film positioned on the electrodes681 and 683 are eliminated. As a result, residual attracting forces donot remain on the electrostatic attracting surfaces. Thus, the wafer iseasily removed from the electrostatic attracting surfaces. The removalof the wafer from the electrostatic attracting surfaces of the hand 68is performed using lift pins, as shown in FIGS. 7 and 8. In removal ofthe wafer from the hand 68, supply of the DC voltages for electrostaticattraction is stopped when the wafer on the hand 68 has arrived at aspecified position and stopped by the vacuum carrying robot 66. At thesame time, the lifting of the lift pins is started when the wafer hasarrived at the specific position and has stopped. Even when the electriccharges stored on the electrostatic chuck are not perfectly eliminatedat the time when the lift pins are brought in contact with the wafer,the wafer is not damaged because, as shown in FIGS. 7 and 8, the push-upforce of the lift pins is adjusted by controlling the action of astepping motor while detecting the push-up force of the lift pins usinga load cell. As a result, the wafer can be removed without lifting thelift pins after an elapse of several seconds (about two or threeseconds) until the electric charges polarized on the positive andnegative electrodes are eliminated after the supply of the DC voltageshas stopped, to thereby improve the throughput in carrying the wafer. Inaddition, if the period of time, several seconds, until the electriccharges are eliminated does not affect the entire throughput of waferprocessing, it is not required to control the push-up force of the liftpins using a load cell.

Further, since the amounts of the electric charges stored on theelectrostatic attracting surfaces of the insulating film are equal toeach other directly before the supply of the DC voltages has stopped,the residual attracting force can be certainly eliminated merely bystopping application of the DC voltages for electrostatic attraction.Accordingly, even in the case of using the electrostatic chuck as thewafer holding portion of the atmospheric carrying robot 62, it cantransfer a wafer on a containing stage in a cassette without a problem.

As described above, according to the sample processing apparatus of thisembodiment, since a wafer can be certainly held on the arm by use of theelectrostatic chuck of the present invention as a wafer holding portionof a wafer carrying robot, the reliability in carrying wafers can beimproved.

Also, since a wafer can be certainly held on the arm, the carrying speedof the carrying robot can be increased, to thereby improve thethroughput. Further, in a case where the electrostatic chuck of thepresent invention is used for the wafer carrying robot provided with ahand having two wafer holding portions on the arm, when a wafer whichhas been processed in a processing chamber is exchanged with anon-processed wafer, the wafer is not removed by centrifugal force evenwhen increasing the turning speed as the wafer is turned from one end tothe other end of the hand, that is, the arm (or the hand) is rotated bythe carrying robot. Accordingly, exchange of wafers in the processingchambers can be quickly performed, thereby reducing any loss of time inwafer processing.

In addition, the atmospheric carrying robot uses an electrostatic chuckin this embodiment, it may adopt a different holding means such as avacuum chuck.

Although the electrostatic chuck, and the method of and apparatus forprocessing a sample using the electrostatic chuck, according to thepresent invention have been described by way of example in the first,second, third and fourth embodiments, the most important point of thepresent invention lies in the fact that in an electrostatic chuckapplied to a sample processing apparatus or a sample carrying apparatus,the amounts of electric charges stored on a dielectric film, directlybefore the supply of a DC voltage applied between positive and negativeelectrodes is stopped, are equal to each other. An electrostatic chuckhaving such a configuration is allowed to smoothly eliminate the storedelectric charges and to substantially prevent generation of a residualattracting force. Additionally, in the sample processing apparatus,represented by a plasma processing apparatus or a vacuum processingapparatus, using the electrostatic chuck, a sample can be certainly heldduring processing or carrying the wafer, or upon delivery of the waferto the next processing chamber, the wafer can be quickly removed fromthe chuck without damage to the wafer, so that it is possible to improvethe working ratio of the apparatus.

As described above, in the electrostatic chuck of the present invention,since the amounts of electric charges stored on electrostatic attractingsurfaces of an insulating film corresponding to the positive andnegative electrodes, directly before stopping the supply of DC voltagesapplied to the electrodes, are equal to each other, the electric chargesstored on the electrostatic attracting surfaces of the insulating filmcan be quickly eliminated without separately providing any electriccharge eliminating step after the supply of the DC voltages has stopped,so that little residual attracting force is generated and the timerequired for eliminating the stored electric charges is shortened.

According to the sample processing apparatus using the electrostaticchuck of the present invention, since little residual attracting forceis generated and the time required for eliminating the stored electriccharges is shortened, lowering of the processing ability of theprocessing apparatus can be prevented. In addition, according to theelectrostatic chuck of the present invention, it takes two or threeseconds to eliminate the stored electric charges. Such a time is notregarded as a loss of time in consideration of the time required foroperating lift pins or the like. However, if needed, the electriccharges stored on the dielectric film can be more quickly eliminated byapplying voltages having polarities reversed with respect to thevoltages for electrostatic attraction after stopping the appliedvoltages.

In particular, according to the plasma processing apparatus using theelectrostatic chuck of the present invention, it is possible toeliminate an unbalance between the amounts of electric charges generatedduring plasma processing, which is performed simultaneously withapplication of a radio frequency voltage for generating a bias voltage,by maintaining generation of the plasma for a specific time afterstopping the application of the radio frequency voltage. Also, it ispossible to eliminate an unbalance between amounts of electric chargesgenerated during plasma processing by applying the DC voltages forelectrostatic attraction for a specific time after plasma extinction.Further, since elimination of electric charges stored on theelectrostatic attracting insulating film after stopping the supply ofthe DC voltages for electrostatic attraction is performed within aprocessing gas exhausting time, it is possible to prevent lowering of aprocessing ability due to the electrostatic chuck.

According to the electrostatic chuck of the present invention,particularly, since a residual attracting force is eliminated afterstopping the supply of the DC voltages, a substrate mounting surface isprevented from being deposited with contaminants having an electriccharge, as compared with a chuck in which there exists a residualattracting force, so that the back surface of a new substrate isprevented from being deposited with contaminants.

In the case where contaminants adhere on the electrostatic chuck of thepresent invention, the contaminants, which adhere on a dielectric film(insulating film for electrostatic attraction) of the chuck, can betransferred on a dummy wafer to be thus removed by applying a voltagehigher than the usual applied voltage between the electrodes forelectrostatic attraction or by applying an AC voltage having an absolutevalue larger than that of the usual applied voltage between theelectrodes. Thus, by periodically repeating the above operation, it ispossible to reduce contaminants adhering on the back surface of aproduct wafer.

Additionally, in the case where the electrostatic chucks of the presentinvention are used for all of wafer holding portions of a sampleprocessing apparatus, since little residual attracting force isgenerated at each of the wafer holding portions, it is possible tocertainly deliver a wafer because the wafer is easily removed from awafer holding portion, and hence the reliability of the apparatus issignificantly enhanced.

Additionally, in the case where a power failure occurs during processingof a wafer held by an electrostatic chuck, the attracting force of thewafer is reduced and it is floated and offset by the pressure of a heattransfer gas remaining on the back surface of the wafer. In this case,the pressure of the heat transfer gas may be reduced while theattracting force of the wafer is maintained. Specifically, when thesupply of the DC voltages to the electrostatic chuck is abruptlystopped, the attracting force may be maintained for a specific period oftime by auxiliary batteries attached to the DC power supplies forsupplying voltages to the inner electrode and ring electrode, and duringthe specific period of time, the heat transfer gas may be exhausted. Oneof the simple methods for exhausting a heat transfer gas is to provide avalve for opening a supply line of the heat transfer gas in the supplyline, thereby connecting the supply line communicating with the backsurface of the wafer into a processing chamber in which the wafer isdisposed when the supply of the voltages is stopped. According to thismethod, when the supply of the voltages is stopped, the heat transfergas flows in the processing chamber, and the pressure at the backsurface is balanced against the pressure of the processing chamber, tothereby prevent the wafer from being offset.

What is claimed is:
 1. An electrostatic chuck comprising:a pair ofelectrodes having different polarities; and a dielectric film, formed ontop surfaces of said pair of electrodes, on which a sample is to beelectrostatically attracted and held when a DC voltage is appliedbetween said pair of electrodes; wherein the product of a ratio betweenelectrostatic capacities between said positive and negative electrodesand said wafer and a ratio between resistances of portions of saiddielectric film on said positive and negative electrodes is set to beapproximately 1, by changing areas and surface roughnesses of actualattracting portions of said dielectric film corresponding to saidpositive and negative electrodes.
 2. A sample processing apparatus forprocessing a sample electrostatically attracted and held on anelectrostatic chuck, said electrostatic chuck comprising:a pair ofelectrodes having different polarities; a dielectric film, formed on topsurfaces of said pair of electrodes, on which a sample is to beelectrostatically attracted and held when a DC voltage is appliedbetween said pair of electrodes; wherein a recess not in contact withthe back surface of said sample is formed in a surface of saiddielectric film on which said sample is disposed; and amounts ofelectric charges of different polarities stored on attracting portionsof the surface of said dielectric film excluding said recess are equalto each other; and wherein the product of a ratio between electrostaticcapacities between said electrodes having different polarities and saidwafer and a ratio between resistances of portions of said dielectricfilm on said electrodes having different polarities is set to beapproximately 1, by changing areas and surface roughnesses of saidattracting portions of the surface of said dielectric film on saidelectrodes having different polarities.
 3. A sample processing apparatusaccording to claim 2, further comprising means for maintainingapplication of the DC voltage between said electrodes for a specifictime after plasma extinction.
 4. A sample processing apparatus accordingto claim 2, further comprising means for removing said sample from saidsample mounting surface without performing any other step after stoppingsupply of the DC voltage between said electrodes.
 5. A sample processingapparatus according to claim 2, wherein said processing of said sampleis plasma processing, and said apparatus comprises means for applying aradio frequency voltage for generating a bias voltage during said plasmaprocessing, and means for maintaining generation of the plasma for aspecific time after stopping supply of said radio frequency voltage upontermination of processing said wafer.
 6. A sample processing apparatusaccording to claim 5, further comprising means for maintainingapplication of the DC voltage between said electrodes for a specifictime after plasma extinction.